An interesting case was that of the MoPac and its “actuarial rebuilds” of circa 1940. For the MT-75 class 4-8-2, it chose Baldwin disc drivers, whereas for the N-75 class 4-8-4, the Boxpok type was used.
From the photographic evidence, the Baldwin disc drivers on the MT-75 were of the later, non-ribbed type. However, Collias (1) stated: “Baldwin’s aesthetically tasteful disc wheel centers with cast ribbed reinforcement between each hole opening were used.”
I’d be circumspect in suggesting that the choice in each case was made for technical reasons. It could well have been “low bidder” or spreading the business around. But if say the Baldwin design was a bit lighter, then it might have had the edge for the MT-75, which would not have had unduly high piston thrust and which was apparently expected to run freely at 90 mile/h.
Again from the photographic evidence, some of the MoPac 1927-30 MT-73 class “big” 4-8-2s were retrofitted with web-spoke main drivers.
In further reading I have seen both the terms “Alco Boxpok” and “GSC Boxpok” used. Whether this is simply different wording choices for the same thing, or whether Alco itself cast some Boxpok drivers is unclear. I have also seen one reference to Baldwin-cast Boxpok drivers. Some more drilling-down is required here. Still, given the extent of cross-licensing practiced in American industry, one would not be too surprised to find others having cast Boxpok drivers.
Cheers,
(1) Joe T. Collias; MoPac Power: Missouri Pacific Lines Locomotives and Trains 1905-1955; Howell-North; 1980; ISBN 0-8310-7117-6.
